Graphic arts applications frequently require the accurate reproduction of a high resolution color image (commonly referred to as an "artwork"), such as a color photograph, a color drawing, a color layout and the like. A typical application might involve printing a high resolution color image or a series of such images on a page of a periodical, such as a magazine, or a corporate annual report.
Images are oftentimes generated either photographically, on suitable film, or electronically, on video tape or other suitable electronic media. When generated, images share a basic characteristic: they are recorded on a continuous tone (hereinafter referred to as "contone") basis. As such, the color existing at any point in the image is represented by a plurality of continuous amplitude values, oftentimes discretized as eight-bit values ranging from "0" to "255".
Color reproduction equipment takes advantage of the principle that the vast majority of colors can be separated into a specific linear combination of four primary subtractive colors (cyan, yellow, magenta and black--C, Y, M and K) in which the amount of each primary color is set to a predetermined amount. In the case of printed reproductions of an image, use of primary color printing obviates the need to use a differently colored ink for each different color in the image. As such, each image is converted into a succession of three or four color separations, in which each separation is essentially a negative (or positive) transparency with an altered tone reproducing characteristic that carries the color information for only one of the primary colors.
Modern offset printing presses do not possess the capability of applying differential amounts of ink to any location in an image being printed. Rather, these presses are only designed to either apply or not apply a single amount of ink to any given location on a page. Therefore, an offset printing press is unable to directly print a contone separation. To successfully circumvent this problem, halftone separations are used instead. An image formed from any single color halftone separation encodes the density information inherent in a color image from amplitude modulated form into a spatial (area) modulated form, in terms of dot size, which is subsequently integrated by the human eye into a desired color. By smoothly changing halftone dot sizes (dot areas), smooth corresponding tone variations will be generated in the reproduced image. Given this, the art has taught for some time that a full color image can be formed by properly overlaying single color halftone reproductions for all of the primary subtractive colors, where each reproduction is formed from a halftone dot separation that contains dots of appropriate sizes and in one of these primary colors. Clearly, as size of the dots decreases, an increasing amount of detail can be encoded in a dot pattern and hence in the reproduced image. For that reason, in graphic arts applications, a halftone separation utilizes very small dots to yield a relatively high dot pitch (resolution).
With this in mind, one might first think that printing a color image for graphic arts use should be a fairly simple process. Specifically, a color image could first be converted into corresponding continuous tone separations. Each of these contone separations could then be converted into a corresponding halftone separation. A printing plate could then be manufactured from each halftone separation and subsequently mounted to a printing press. Thereafter, paper or other similar media could be run through the press in such a fashion so as to produce properly registered superimposed halftone images for all the subtractive primary colors thereby generating a full color reproduction of the original image.
In practice, accurately printing a color image is oftentimes a very tedious, problematic and time consuming manual process that requires a substantial level of skill. First, the conventional manual photographic process of converting a contone separation into a halftone separation, this process commonly being referred to as "screening", is a time and resource consuming process in and of itself. Second, various phenomena, each of which disadvantageously degrades an image, often occur in a reproduced halftone color image. Moreover, the complete extent to which each of these phenomena is present in the reproduced image is often known only at a rather late point in the printing process thereby necessitating the use of tedious and time and resource consuming iterative trial and error experimentation to adequately eliminate these phenomena.
Specifically, to verify the accuracy of the color printing process and to enable appropriate adjustments to be made at various stages in the printing process in order to correct image defects and improve reproduction accuracy, a test image, frequently referred to as a "proof" is generated from the halftone separations once they are made. After a proof is generated, it is presented as being representative of the reproduced image which will be produced by a printing press in order to determine the accuracy of the printed image. Oftentimes, the proof contains unexpected and unsightly Moire patterns that arise from the interaction of pattern(s) in the image itself with that introduced by use of angled halftone screens that are used to photographically generate the halftone separations. Frequently, these Moire patterns can be rendered invisible by rotation of one or more of the screens to a different screen angle. Unfortunately, the exact change in the screen angle is frequently very hard to discern from the resulting Moire pattern itself and instead must be determined through trial and error experimentation. Many other unexpected artifacts can also exist in the proof thereby necessitating that various changes must be made to one or more of the separations. As such, this requires that one or more new halftone separations must be generated or at least changed, a new proof must be produced and then analyzed, with this "proofing" process being iteratively repeated until the objectionable Moire patterns and all objectionable artifacts are eliminated from the proof. Now, once an acceptable proof is made, thereby indicating that a printed image based on the separations will likely present a desired depiction of the original artwork, a separate printing plate is then made for each halftone separation. At this point, a full color print, commonly referred to as a "press sheet", is produced from these plates onto a sheet of actual paper stock that is to be used to carry the reproduced image. The press sheet is then examined to discern all imperfections that exist in the image reproduced therein. Owing to, e.g., unexpected tone reproduction shifts, the existence of any artifacts in the press sheet and tone variations occurring between the press sheet and the desired image of the original artwork, further adjustments in the coloration or screen angle of the separations may need to be made with the entire process, i.e. generation or modification of halftone separations and printing plates, being repeated until an acceptable press sheet is produced. With experience gained over several years, a skilled color technician can reduce the number of times that this entire process needs to be repeated in order to produce a set of color halftone separations that yields an acceptable press sheet.
As one can now readily appreciate, the iterative manual process of producing an acceptable set of halftone separations, due to the inherent variability of the process, can be very tedious and inordinately time consuming. Unfortunately, in the graphic arts industry, publication deadlines are often extremely tight and afford very little, if any, leeway. Consequently, the available time in a graphic arts production environment allotted to a color technician to generate a set of halftone separations to meet a particular publication deadline, for example, is often insufficient to allow the technician adequate time, due to the trial and error nature of iterative process, to generate that set of separations which produces a very high quality halftone color image. As such, the technician is often constrained by time pressures to produce a set of separations that produces a visually acceptable and hence satisfactory, though not necessarily a very high quality, image.
In addition, the manual process can be disadvantageously quite expensive. Inasmuch as the manual process, even for a skilled color technician, involves a certain amount of trial and error experimentation, a number of separate proofs is often made with changed or new separations being generated as a result. Each new separation requires another piece of film. Film and associated developing chemicals are expensive. In addition, if an unacceptable press sheet is produced, then additional separations may need to be made along with new printing plates, which further lengthens the process and increases its expense.
In an effort to reduce the time required and expense associated with conventional manual photographic based color reproduction processes, the art has turned away from use of these manual processes in high volume graphic art applications to the use of intermediate off-press proofing technologies, such as electro-photographic techniques. In this regard, U.S. Pat. No. 4,708,459 (issued to C. Cowan et al on Nov. 24, 1987, assigned to the present assignee hereof and hereinafter referred to as the '459 Cowan et al patent) discloses an electro-photographic color proofing system. While this system generally produces an excellent quality proof, it does not permit a user to precisely specify a tone reproduction curve shape which, in turn, restricts the ability of this system to provide accurate tone reproduction over the entire operating space of the system.
Specifically, tone reproduction, as it relates to a digital separation, essentially defines an input/output relationship between measured optical reflection densities and corresponding, e.g. eight-bit, contone values. To provide accurate tone reproduction, the measured densities should properly track the contone values. Each contone value represents a corresponding area of a halftone dot. As will be seen, tone reproduction entails consideration of two phenomena: do gain and solid area density.
As to dot gain, it has been known in the art for quite some time that the effective area of a halftone dot, as printed and as perceived by a viewer, frequently diverges from that of its associated contone value. This is generally caused by a number of factors, some of which are strictly physical in nature, such as for example media absorbency and ink spreading, while others are optical in nature, such as an amount of light that is reflected from or absorbed into a dot. As the actual dot areas that form an image diverge from the corresponding contone values these areas are intended to depict, that image exhibits increasingly poor tone reproduction. For ease of reference and convenience, optical reflectance densities for halftone images are generally viewed, in the art, in terms of equivalent halftone dot areas which permits tone reproduction to be considered in terms of dot gain.
Dot gain, as that term is known in the art, is simply defined as the difference, expressed as a percent and referenced to a full sized (100%) halftone dot, between the size of an output halftone dot (i.e. "dot out") that is produced for a given sized input halftone dot (i.e. "dot in"), specifically dot gain=dot out-dot in. When graphically depicted over a full range of input dot areas, i.e. from 0-100%, output dot area typically does not equal input dot area on a 1:1 basis. While 0% and 100% sized input dots must correspondingly produce 0% and 100% output dots, i.e. zero dot gain must occur at both of these limit points, dot gain is frequently non-zero and positive between these limits thereby defining a non-zero tone reproduction curve. If output dot area were to equal input dot area on a 1:1 basis throughout the entire range, then this would define a 45.degree. line (for null dot gain) that emanates from an origin on a dot in vs. dot out curve. Corresponding numerical values for dot gain are frequently specified at 25%, 50% and 75% sized input dots, and occasionally for input dots sized at 90% (for shadow areas) and 10% (for highlight areas). A simple dot gain curve may take the shape of an inverted parabola that has zero dot gain for a 0% input dot, increases on a non-linear basis through 10% and 25%, reaches a maximum at 50%, and thereafter non-linearly decreases through 75% and 90% and finally once again reaches zero at a 100% input dot. The shape of the dot gain curve for any halftone separation effectively provides the shape of the tone reproduction curve of that separation.
Solid area density, frequently stated as D.sub.max, simply defines the optical reflection density of a solid area that is to be produced by a halftone separation for the maximum contone value associated therewith, e.g. "255" for eight-bit contone values. The value of D.sub.max effectively scales the tone reproduction curve of the image formed by that particular separation.
It has been known for some time in the art, that maximum solid area density and dot size are physically linked. In this regard, even as the physical area (i.e. the actual coverage) of a halftone dot remains constant, the apparent size of this dot (i.e. that perceived by a viewer and owing to the optical effects of light reflectance and absorbance between the dot and the media on which that dot is printed) varies with changes in solid area density. Furthermore, the optical effects of tone reproduction will often be exacerbated by various physical effects, such as media absorbency or ink spreading as noted above, that are associated with actually printing these dots. Specifically, by virtue of these physical effects, a larger or smaller dot may actually be printed than that which was intended.
With the above in mind, if a color proof image is to match the tone reproduction inherent in a press sheet, then that proof image needs to accurately reproduce both a desired solid area density and a dot gain curve shape that are expected to result in the press sheet for each primary color separation in the proof.
To effectuate some control over dot gain, the electro-photographic proofing system described in the '459 Cowan et al patent, permits an operator to specifically vary dot size of the halftone dots at a 50% input dot size for any separation. However, this system does not permit the operator to precisely specify a desired dot gain curve shape to be used to generate a primary color halftone image from that separation. By only providing such dot size control, this system, in effect, merely allows an operator to select any one dot gain curve from a family of pre-defined and similarly shaped dot gain curves that only vary amongst each other by scale. However, mere selection among a family of pre-defined dot gain curves oftentimes does not result in a proper dot gain curve shape that accurately reflects the dot gain inherent in a color printing process which the proof image is to represent. Consequently, the proof will not accurately exhibit the dot gain curve shape inherent in that process. As such, the very limited control over dot gain provided by electro-photographic proofing systems has tended to unduly limit the tone reproduction capability of these systems and hence has often prevented these systems from accurately reproducing various subtleties in a color proof image that would appear in a press sheet.
For a variety of reasons, such as for example, increased flexibility, control and throughput over that provided by optical (including electro-photographic) proofing systems, the art is currently turning towards the use of so-called direct digital color proofing (DDCP) systems. These systems directly generate a halftone color proof image from a set of digitized contone separations and particularly the digitized contone values therefor. Specifically, DDCP systems manipulate the separations in digital form to electronically generate appropriate halftone separations, including, inter alia, electronic screening and tone reproduction compensation, and then directly write the proof image using an appropriate high resolution binary marking engine. Furthermore, inasmuch as these systems completely eliminate photographic film based processes, these systems are expected to be very economical to operate.
By virtue of providing dot gain compensation in a completely digital fashion, these DDCP systems will permit far better control over image subtleties and hence tone reproduction than that available through optical proofing systems known in the art.
In that regard, I have previously developed a technique for inclusion in, illustratively, a DDCP system that allows an operator to completely specify and readily change a desired dot gain curve shape that, within the physical limits of the system, is to be reproduced in the proof and then have the system produce a proof image that exhibits the desired dot gain curve shape. That technique is fully described in my co-pending United States patent application "A TECHNIQUE FOR USE IN CONJUNCTION WITH AN IMAGING SYSTEM FOR PROVIDING ACCURATE TONE REPRODUCTION IN AN OUTPUT IMAGE" filed Oct. 25, 1991, U.S. patent application Ser. No. 07/782,940 and which has been assigned to the present assignee hereof.
Very broadly speaking, this technique relies on intentionally varying the value of each incoming contone value by an amount consistent with both the actual tone reproduction characteristic of a DDCP imaging chain (i.e. a so-called "Process" dot gain) and a desired (so-called "Aim") dot gain to yield an output dot of an appropriate area that provides the desired density in the proof image. In this context, the DDCP imaging chain is illustratively formed of a raster image processor (RIP), which implements a screening process, and a marking engine connected thereto such as a sublimation dye transfer laser writer. To readily accomplish this variation, all the incoming contone values are appropriately modified through illustratively a table look-up into correspondingly modified values which, when subsequently rendered into halftone patterns on the proof image by the marking engine cause the proof to accurately exhibit the desired "Aim" tone reproduction curve. The look-up table contains values which represent the "Aim" tone reproduction curve modified by an inverse of the "Process" tone reproduction curve.
While this technique yields excellent results, it requires that the "Process" tone reproduction curve be accurately specified at the specific operating condition at which the DDCP imaging chain is to operate. In that regard, it is well known that the response of a halftone imaging chain will vary based upon a number of factors, including, though not limited to, variations in, illustratively, screen ruling, color, dot font and solid area density.
I have observed that, owing to thresholding inherent in a sublimation dye transfer DDCP system, particularly involving dye transfer response to exposures from the writing lasers used therein, the size of a halftone dot, at any screen ruling, exhibits an approximately linear variation with changes in solid area density of that dot. For example, for a 50% input dot written at a screen ruling of 150 lines/inch (1 pi) (approximately 60 dots/cm), a change in solid area density of 0.8 (from a low to a high density as measured in "Status T" units) causes an apparent 10% increase in dot size. At screen rulings of 200 1 pi (approximately 79 dots/cm) and 120 1 pi (approximately 47 dots/cm), the dot size variation for the same 50% input dot amounted to 14% and 6%, respectively. Furthermore, at any solid area density, dot size, being primarily a cumulative perimeter effect over all the dots in a region of the image, exhibits an approximately linear variation with corresponding changes in screen ruling. For a 50% input dot, a change in screen ruling from 150 to 200 1 pi, or 150 to 100 1 pi appears to respectively cause a 2% increase or 2% decrease in tone reproduction. While screen ruling induced tone reproduction variations are significantly smaller than solid area density induced variations, both variations in tone reproduction are noticeable, objectionable and best avoided.
To fully account for such performance variations, my prior inventive technique required that once an operating condition was completely specified, a test proof image had to be made, typically using a null dot gain look-up table, at that particular condition and then densitometrically measured. The measurements yielded the "Process" tone reproduction curve which was then used in conjunction with the desired "Aim" tone reproduction curve to construct appropriate values for the look-up table.
As one can readily appreciate, each time the operating condition changed by an amount which would lead to an objectionable tone reproduction change, unfortunately a new test proof had to be made and measurements taken thereof. In the case of density changes at 200 1 pi, an objectionable change could result from as little as a 0.1 change in output density. Not only does a proof image consume imaging media, which is fairly expensive, but more importantly, it consumes time both of a DDCP system and its operator. Typically, while a proof image may require upwards of approximately 15 minutes of machine time to produce, it may consume upwards of 30 minutes for a skilled operator to properly measure.
Since, as noted, publication deadlines in the graphics arts industry often afford very little, and often no appreciable, leeway in time, an operator generally does not have the available time to make and properly measure a test proof whenever he changes the operating condition of the DDCP imaging system. Thus, if he were to use my prior inventive technique, as described above, but not make a test proof, he would be constrained to either operate the DDCP system at only known operating conditions, for which test proofs have been previously made and measured, or vary the operating condition as desired but accept the ensuing performance variations in tone reproduction with a potentially adverse effect on resulting image quality--i.e. the proof image may not accurately represent an image that would be depicted on a resulting press sheet.
Thus, a need now exists in the art for a technique, which can be used in conjunction with a DDCP system, that can significantly reduce the number of test proof images that needs to be made in order to produce a "Process" tone reproduction curve that accurately characterizes the native response of an associated DDCP imaging chain for any change in the operating conditions, e.g., in solid area density and screen ruling, throughout the entire operating space of this chain. Specifically, such a technique should not require that a separate test proof be made and measured for each change in the operating condition.